Several techniques and systems have been developed for correcting and stabilizing the spine and for facilitating fusion at various levels of the spinal column. One form of a spinal fixation system utilizes bendable spinal rods situated on opposite sides of the spine and coupled to a plurality of fixation elements, which are in turn attached to various portions of the spine. One such system is described in U.S. patent application Ser. No. 09/408,197, entitled "Advanced Coupling Device Using Shape-Memory Technology" and filed on the same day as the subject application, which is hereinafter incorporated by reference. This particular system includes a coupling element defining a longitudinal passage therethrough and having a side wall defining first and second slots extending from opposite ends thereof and including proximately adjacent and longitudinally overlapping portions. A locking element at least partially formed of a shape-memory material is positioned about the coupling element and, upon a sufficient change in temperature, contracts about the coupling element and compresses the side wall against a member disposed within the longitudinal passage to limit movement of the member relative to the coupling element.
Shape-memory materials are alloys of known metals, such as, for example, copper and zinc, nickel and titanium, and silver and cadmium. These alloys are known to exhibit a "shape-memory" characteristic in which a particular component formed of a shape-memory alloy ("SMA") is capable of reforming to a "memorized" shape upon a sufficient change in temperature. Such components are originally deformed from an initial configuration and retain their deformed configuration until they are sufficiently heated, at which point they begin to recover toward their original, memorized configuration. This recovery phenomena occurs when the SMA alloy changes from a martensitic crystal phase to an austenitic crystal phase. SMAs thus exhibit a memory effect upon passing from a low temperature form (e.g. martensitic stage) to a high temperature form (e.g., austenitic stage).
When an SMA is heated, transformation toward the original, memorized shape occurs over the range A.sub.s,-A.sub.f, where A.sub.s, and A.sub.f are the temperatures at which formation of austenite begins and is completed, respectively. Conversely, when an SMA is cooled, transformation back toward the deformed shape occurs over the range of temperatures M.sub.s,-M.sub.f, where M.sub.s, and M.sub.f are the temperatures at which formation of martensite begins and is completed, respectively. For many SMAs, A.sub.s, and M.sub.s are approximately the same and therefore shape transformation begins at about the same temperature, independent of whether the SMA is being heated or cooled.
In certain medical applications, it is often beneficial to choose a SMA having an A.sub.f temperature less than normal body temperature to avoid having to provide external means for applying heat to raise the temperature of the SMA above A.sub.f. Additionally, because it is necessary for the SMA to remain in the austenitic state during use (i.e., to avoid possible re-expansion of a SMA coupling device which may result in the loosening of clamping forces), the M.sub.s of the SMA is preferably somewhat lower than the lowest temperature which the SMA will encounter during use. It is therefore advantageous to choose a SMA with relatively low A.sub.f and M.sub.s temperatures to: 1.) ensure that the maximum stress is recovered from the SMA by completely transforming the shape-memory material to its austenitic state, and 2.) avoid potential problems which may result if the temperature of the SMA drops below the M.sub.s temperature. However, by choosing an SMA with a relatively low A.sub.f temperature, there is a risk that the SMA will become sufficiently heated before or during installation so as to result in the premature transformation of the SMA into the austenitic state (i.e., the premature locking of a SMA coupling device before or during a surgical procedure). In the past, this typically required providing an external cooling means to maintain the temperature of the SMA below A.sub.f before and during installation of the shape-memory device.
In some applications of shape-memory alloys, mechanically constrained shape change may be preferable over heat actuated shape change. Mechanically constrained shape change employs the unique characteristics associated with stress-induced martensite, which does not require a change in temperature to effect reformation of the shape-memory material toward its original, memorized configuration. A mechanical restraint is initially used to hold the shape-memory alloy material in its deformed configuration (in a stress-induced martensitic state) until such a time when the restraint is removed, thus allowing the shape-memory alloy to reform toward its original, memorized configuration without a change in temperature. The use of stress-induced martensite (utilizing mechanically constrained shape change) in place of conventional shape memory alloy materials (utilizing heat actuated shape change) often offers a greater degree of control over the reformation of the shape-memory material. Stress-induced martensite also offers easier alloy composition control, reduces mating part tolerance requirements, and simplifies mechanical reversal at minimal stress levels. Further details regarding the use and characteristics of stress-induced martensite are more fully described in U.S. Pat. No. 5,597,378 to Jervis, entitled "Medical Devices Incorporating SIM Alloy Elements", which is hereinafter incorporated by reference into the subject application.
Thus, there is a general need in the industry to provide a device and method which selectively prevents the locking of a shape-memory alloy coupling system and allows for manipulation of the coupling system. The present invention meets this need and provides other benefits and advantages in a novel and unobvious manner.